Tuning system



y 5, 1942- L. E. BARTON 2,281,661

TUNING SYSTEM Filed Jan. 31, 1941 0.56. run 0 WWI 5 Al? Ihwentor BB Q 5 z Gttorneg Patented May 5, 1942 TUNING SYSTEM Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of Delaware of America, a corporation Application January 31, 1941, Serial No. 376,873 6 Claims. (Cl. 250-20) This invention relates to tuning systems for radio signal reception at relatively low frequencies, such as in a band just above audibility, and has for its object to provide an improved tuning system for attenuating intermediate and image frequency signals in a low frequency signal band of this character, an intermediate frequency being provided which is higher than said signal band by a frequency difference permitting effective intermediate and image frequency attenuation in and tuning of one or more of the main signal input circuits of the system.

The invention will, however, be further understood from the following description when con sidered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

Referring to the drawing, matic circuit diagram of the the figure is a schesignal input portion of a superheterodyne receiving system embodying the invention.

This circuit may include a suitable converter tube 5 having a signal input grid 6 and an oscillator grid 7, together with the usual output anode 8 and a cathode 9. Signals are applied to the signal input grid 5 through a coupling system IB, hereinafter described, from a suitable signal source such as antenna and ground terminals II and 12, respectively.

Oscillations are applied to the converter 1" through the oscillator grid 1 and an input lead I3, from an oscillator tube l4 provided with a suitable variably tunable oscillator circuit IS. The circuit shown is representative of any suitable tunable oscillator circuit.

In this system, however, the oscillator is tuned at all times above the selected intermediate frequency and the intermediate frequency is higher than the signal receiving band. For example, assuming a signal in a low frequency band between 14 and 32 kc., an intermediate frequency of 80 kc. may be used and the oscillator circuit l5 may be tunable over a band of 94 to 112 kc.

The intermediate frequency signals are derived from the output anode 8 through a suitable intermediate frequency coupling transformer l6 connected with the output circuit IT. The remainder of the receiving system does not concern the invention, and hence is not shown.

This system is adapted for receiving relatively low frequency signals, for example, only slightly above the audible or sound frequency range, and the oscillator l4l5 also operates in a comparatively low frequency range, whereby the circuit is highly stable.

cut frequency band may have a added capacity The intermediate-frequency is above the signal receiving range by a frequency difference permitting full tuning and intermediate frequency and image frequency attenuation in the coupling system Hi between the signal input terminals ll-IZ and the signal grid- 6 of the converter tube 5.

This circuit comprises an input coupling transformer 20, preferably of the iron core type, as indicated, having a primary winding 2! connected through a transmission line 22 with the terminals ll l2, and having a low impedance secondary 23 tuned within or adjacent to the signal receiving band by a shunt capacitor 24. One side of the secondary is grounded as indicated at 25 and through ground is connected with one side of a variable tuning capacitor 26, which for the prescapacity variation range of from mmfd. to 1000 mmfd.

} Between the high potential terminal 2'! of the the variable tuning capacitor. 26 connected in shunt to the series circuit.

The tuning of the circuit I0 is accomplished by the tuning capacitor 26 in shunt with the inductance of the serially connected windings 23,

28 and 29. The natural resonance frequency of the inductance 28 with distributed or a small 30 in shunt thereto is of the order of the intermediate frequency, and the inductance 29 with distributed or small added capacity 3|, in shunt thereto, is resonant in the image frequency band, which in the present example is between 1'75 and 192 kc. A resistance indicated at 32 may be added across the inductance 29 to increase the: attenuation and to broaden the rejection band for the image frequencies.

Signal output from the circuit I0 is provided by the tube 5 connected across the capacitor 26 through a grid coupling capacitor 33. The grid 6 is provided with a grid resistor 34 having a bias supply connection 35 with the remainder of the receiving system. The cathode 9 is connected to ground 36 through a suitable bias resistor 31.

The tuning capacitor 24 connected across the input winding 23 serves also to maintain a more constant gainthrough the input circuit as the tuning is varied by the variable capacitor 26. The potential drop through the inductances 28 and 29 increases with frequency and since the total inductance drop must substantially equal that 2 across the capacitor 26, for any tuning adjustment at resonance with a signal, thepotential drop across the input winding 23 must decrease with increase in frequency, and this is accomplished by the shunt capacitor 24 for the winding 23.

By utilizing an intermediate frequency which is relatively highwith respect to the signal receiving band, the inductance 23 may be made relatively small with respect to the inductances 28 and 29, thereby permitting the latter inductances to resonate in the intermediate and image frequency ranges. The total over-all inductance in the input circuit may, therefore, be low enough so that it is tunable throughthelow frequency signal receiving band, while, at the same time, providing intermediate and image frequency attenuation and substantially constant gain at the signal input grid of the amplifier or other tube connected with the circuit.

It will be appreciated that while the invention has been described in connection with a single input circuit, such as that; for coupling an antenna to a signal amplifier or mixer tube, the tuning and attenuating means may be provided in a plurality of circuits in the same manner as shown and described.

I claim as my invention:

1. In a low frequency superheterodyne tuning system, a tunable signal conveying circuit comprising a signal input inductance, a gain controlling and tuning capacitor connected in shunt therewith, an intermediate frequency attenuating inductance andan image frequency attenuating inductance serially connected in circuit with said first namedinductance, a tuning capacitor connected in shunt with said series connected inductances for tuning said circuit in a frequency range lower than the intermediate frequency, means for deriving an output signal across said last named capacitor, and means for applying an input signal to said first named inductance.

2. In a superheterodyn signal receiving system, the combination of a converter stage, a low frequency signal conveying circuit coupled thereto, an intermediate frequency output circuit coupled to said converter stage and tuned to an intermediatefrequency which is higher than the frequency-response range of said signal conveying circuit, a series of tunable inductance elements providing intermediate and image frequency attenuation in said first named circuit, and means including said inductance elements in series for tuning said signal conveying circuit through said response range.

3. In a superheterodyne signal receiving system, the combination of a tunablesignal conveying circuit comprising a signal input inductance and two relatively high inductance windings serially connected therewith to provide tuning inductance means in said circuit, one of said windings being resonant at an intermediate frequency higher than the tuning band of said circuit and the other of said windings being resonant in a predetermined image frequency band, and means for tuning said input inductance and series connected windings in series jointly through said tuning band.

4. In a superheterodyne signal receiving system, the combination of a signal input circuit therefor comprising a low inductance signal input winding and two high inductance windings serially connected in said circuit, means for tuning said serially connected inductance windings through apredetermined signal receiving range above and adjacent to the range of audible frequencies, and means connected with said input circuit for deriving an intermediate frequency whichis relatively high with respect to the tuning range of said input circuit, means for effecting intermediate and image frequency attenuation in said higher inductance windings, and means for tuning said windings in series through said predetermined frequency range.

'5. A tuning system comprising, in combination, an input coupling transformer having a low impedance secondary, means providing shunt tuning and gain controlling capacity for said secondary, a variable tuning capacitor, a pair of inductance elements connected serially with the secondary across said last named capacitor and being tunable thereby through a predetermined low frequency signal band, and the inductance and distributed capacity of said series connected inductances being such that image and intermediate frequency signals are attenuated thereby.

6. In a superheterodyne signal receiving system, the combination of a low frequency signal conveying circuit, a converter stage coupled to said circuit, an intermediate frequency output circuit coupled to said converter stage and tuned to an intermediate frequency which is higher than the frequency response range of said signal conveying circuit, a signal input inductance, an intermediate frequency signal attenuating inductance, and an image frequency attenuating inductance'in series with the inductance tuning element of said circuit, means connected with said inductances for tuningsaid signal conveying circuit through a predetermined low frequency response range, means for tuning the intermediate frequency attenuating inductance to the intermediate frequency of said receiving system, attenuating inductance within a predetermined range of image frequencies for said system, and a tuning and gain controlling capacitor connected in shunt with the input inductance.

LOY E. BARTON.

means for tuning the image frequency 

